This invention relates generally to satellite-based telecommunications systems and, in particular, to the construction and operation of satellite system user terminals.
The overall capacity performance of a mobile satellite telecommunications system can be enhanced by reducing, for a given link power level, the average transmitted data rate. The use of a selective data rate transmission to and from the users is a technique which enables the system to maintain a given user in a connected state during periods of high demand. However, in typical digital communications systems this implies that voice quality is reduced during this period. This is particularly true of mobile satellite communications systems where link margin is a critical concern.
Modern satellite and terrestrial telecommunications systems typically use some type of digital voice encoder with and without compression. The encoder may be embodied as a codec and may include a vocoder for extracting spectral information from speech and compressing and encoding the extracted spectral information. After encoding and compressing, the speech is further modulated and then is transmitted to a receiver where a decoder reconstructs the original speech signal with the use of one or more Digital Signalling Processors (DSPS). The goal of low bit rate vocoders is to efficiently compress and code the speech signal such that it can be transmitted at a bit rate (e.g., 9.6 kbps or less) that is significantly less than what would be required using conventional analog to digital coder/decoder (codec) techniques (e.g., 64 kbps or 32 kbps). As the vocoder rate is reduced there is typically a user-perceptible reduction in the quality of the reconstructed speech at the receiver.
In a satellite transponder-based communications system, such as a geostationary (GEO) or a non-geosynchronous (e.g., a low earth orbit (LEO) or medium earth orbit (MEO)) system, satellite power is an important and critical system resource. In the forward link, from the system to the user, some amount of satellite power is consumed for each user added to the system, with the actual amount of consumed power being a function of the users data rate, the presence of RF obstructions, the presence of external interference, any diversity technique utilized, and other factors. As the data rate for each user is increased more satellite power is required to service an existing pool of active users, thus fewer new users can be serviced by the satellite, resulting in a decrease in overall system capacity.
On the return link, from the user to the system, power is consumed from the user terminal. In particular, hand held user terminals are sensitive to power usage due to battery operation. System capacity for the return link for these mobile systems can be limited by self-interference due to a plurality of user terminals requesting access to, and using, the system. As more users are added to the system, the self-interference increases, causing a power increase in the individual user terminals to overcome the self-interference. The aggregate of the transmitted power of the user terminals can be reduced by decreasing the transmitted data rate from all or a portion of the user terminals. The reduction in data rate thus decreases the power transmitted by the users and thence the self-interference, therefore the system can support an increase in the number of users supported on the return link.
It is a first object and advantage of this invention to provide an improved telecommunications system that overcomes the foregoing and other problems.
It is a further object and advantage of this invention to provide a satellite communications system that employs a plurality of different rate vocoders, and that has an ability to switch between the plurality of vocoders based on one or more criteria, on one or more links simultaneously.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention, wherein a satellite communications system employs user terminals and gateways each having, in addition to other equipment, a set of vocoders and codecs comprised of at least two vocoders and codecs. In a presently preferred embodiment of this invention the user terminal has at least two vocoders/codecs implemented in one or more circuits, digital devices, or Application Specific Integrated Circuits (ASICs), and a mechanism to switch between the vocoders/codecs for enhancing a user""s ability to hear intelligible speech when operating in the clear or in fading and/or interference environments. A terrestrial gateway detects a user""s current communication condition due to fading and signal blockage and/or interference and selects one vocoder/codec from the set of vocoders/codecs that will maximize the user""s ability to receive intelligible speech. Alternatively, the system may select a vocoder/codec from a list of available vocoders/codecs for all or a subset of a plurality of user terminals. In another embodiment a single vocoder may have multiple individually selectable encoders and decoders.
In another embodiment multiple vocoders may be selected which are connected to a single codec. It is also within the scope of the teaching of this invention to have multiple vocoders which are composed of multiple individually selectable encoders and decoders which are connected to single or multiple codecs.
A user terminal in accordance with an embodiment of this invention includes a transceiver having an RF module for transmitting modulated RF signals to a terrestrial gateway on the forward link and for receiving modulated RF signals from the gateway via at least one satellite on the return link. The at least one satellite may be a geosynchronous or a non-geosynchronous orbit satellite. The RF signals are modulated to convey speech and signalling information.
The user terminal further includes a signal processor coupled to the RF module for demodulating the received RF signals to digital signals, an audio unit for transponding speech signals with a user, and, in one embodiment of this invention, a plurality of vocoders/codecs coupled between the signal processor and the audio unit for encoding and decoding speech signals passing to and from the audio unit.
At the gateway the equipment includes signal processors coupled to transceivers for modulating and demodulating signals to and from the user units to digital signals and a plurality of vocoders/codecs coupled between the signal processor and the audio unit for encoding and decoding speech signals passing to and from a communications network, such as the PSTN. The vocoders may be of the fixed rate or variable rate type.
In accordance with an aspect of this invention a first one of the plurality of vocoders is a variable rate vocoder, a second one of the plurality of vocoders is a fixed rate vocoder, and the fixed rate vocoder operates at an average data rate that differs from, e.g., is less than, an average data rate of the variable rate vocoder. The fixed rate (or the variable rate) vocoder can thus be selected for use in order to conserve satellite power and to maintain or increase system capacity. The fixed rate vocoder may also operate at a higher average data rate than the average rate of the variable rate vocoder.
The user terminal further includes a controller, responsive to signalling information received from the gateway, for selecting at any given time only one of the plurality of vocoders/codecs for coupling between the signal processor and the audio unit. Likewise the gateway can select for use one of the plurality of vocoders/codecs in its own set of equipment.
The gateway has decision making capability which autonomously, or in response to network instruction, can select encoder/decoders or vocoders to be used on either one of the forward or return links or both.
The selection is made in the gateway based on one or more criterion. The one or more criterion can be a presence of an RF signal impairment within the communication link, a presence of a source of external RF interference, a predicted communications system or channel load, an actual communications system or channel load, a user selected voice quality, and/or a signal quality received from the user terminal. In one embodiment the vocoder selection command is generated in the terrestrial gateway in response to a request from the user terminal, and in this case the criterion is based on a quality of a signal received from the terrestrial gateway at the user terminal. In another embodiment the gateway receives instruction from a network system controller or ground operation control center for optimizing the communications within a network of a plurality of gateways. The selection can also be made based on an optimization of communication link performance, an optimization of a number of active user terminals, a desire to extend user terminal talk time and battery life, or to reduce or otherwise control power within a given satellite beam.
As was indicated above, in a further embodiment of this invention a single vocoder having multiple individually selectable encoders and decoders may be used.